In know they're gonna be super tiny (1-3mm) but then they're also Solenopsis which usually like to walk a lot, so should I do rather long tubing between the nest and the outworld or shorter tubing because they're so small? What is a good tubing length (also temperature-grade-wise)?Or should I offer them a long and a short route? (my outworld will have two plugs as will the nest)

I'd say it depends on the size of the colony, if you're getting a queen with like 10 workers, i'd keep the tubes short so they will forage there and not walk a while and give up, and as the colony gets bigger extend the tubing.

My plan for the start is to put the (darkened) test tube into the outworld so they're gonna find food basically in front of their door. When they're about 40-50 workers I'm gonna attach the nest and wait until they want to move into the nest by themselves (which will probably take a few weeks if not months). I guess when they move to the nest they're already gonna have a good colony size - so that probably means a bit of a longer tubing.